The present invention relates to an electrorheological fluid. More specifically, it relates to an electrorheological fluid which has excellent properties in that a large shear stress and low current density can be generated even when a relatively low electric field is applied to the fluid and the generated shear stress and current density has excellent stability properties with time. The invention also has excellent properties in regard to dispersion stability under the conditions of no electric field (the ability to uniformly maintain an electrorheological fluid and not settle or float the dispersed phase therein), re-dispersibility (the ability to duplicate a uniform electrorheological fluid by applying a simple external force when the dispersed phase therein has settled and floats in a non-uniform state), and fluidity (having a low viscosity under the conditions of no electric field). 2. Description of the Prior Art
There are known, as electrorheological fluids generating high shear stress, a fluid in which a powdered ion-exchange resin is dispersed in the higher alkyl ester of an aromatic carboxylic acid (Japanese Kokai (Japanese patent Laid-open) 50-92278), a composition composed of a crystalline material which conducts current only along one of the three crystalline axes, a dielectric fluid, and suspension stabilizing agent (Japanese Kokai 1-170693), and a fluid using, as a dispersed phase, a conductive particle covered with a film layer of insulating material (Japanese Kokai 64-6096). However, these electrorheological fluids have the disadvantages of poor dispersion stability under the conditions of no electrical field, poor re-dispersibility after sedimentation and floatation, and poor fluidity when the concentration of the dispersed phase is increased.
In order to improve the dispersion stability, there have been proposed the use of modified polysiloxane as an additive (Japanese Kokoku 3-39560), the use of a polymer having an ester group and an aromatic group structure as the additive (Japanese Kokai 4-96997) or the like. However, these additives serve only to prevent the sedimentation of the dispersed phase, but have the drawback of a reduction of the re-dispersibility after sedimentation.
Further, in order to improve the re-dispersibility, there have been proposed electrorheological fluids using fine particles as the additive (Japanese Kokai 3-160094 and Japanese Kokai 3-166295) or the like. However, these electrorheological fluids have problems in that the shear stress obtained when an electric field is applied, decreases due to the addition of the fine particles, the conditions of use for the electrorheological fluids are defined due to the poor dispersion stability, and it is necessary for the device used to be equipped with an additional re-dispersibility system.
An object of the present invention, therefore, is to provide an electrorheological fluid.
Another object of the present invention is to provide an electrorheological fluid which has excellent properties in that even when a relatively low electric field is applied to the fluid, a large shear stress is generated while the current density is small, and the generated shear stress and current density exhibit excellent stability with time. The invention also has excellent properties in regard to dispersion stability under the conditions of no electrical field (the ability to uniformly maintain an electrorheological fluid and not settle or float the dispersed phase therein), re-dispersibility (the ability to duplicate a uniform electrorheological fluid by applying a simple eternal force when the dispersed phase has settled and floated in a non-uniform state), an fluidity (having a low viscosity under the conditions of no electric field).